For the itching dog, it could either be an allergy to a grass or some other plant around her, or an allergy to some biting parasite on her. If she is quite a short dog, allergy to grasses could be a big part of the problem. Is she a shorter dog than your other dogs?
I'd try just plain jojoba oil, but you may have to put up with oily spots where she lies down (I suppose you had vasoliney spots last summer?) It won't matter if she licks the jojoba, if anything it will help give her a shiney coat
Another idea is oatmeal water before the putting the jojoba on, she'd probably quite enjoy having her belly bathed with oatmeal water in the heat! The following is off a skincare website for humans, but it's safe for dogs too....
"Oatmeal Water is obtained by immersing an oatmeal stocking in warm water and allowing the mucilage from the oats to seep out.
Firstly, purchase inexpensive stockings from the supermarket then place a hand inside one of the legs, grab a handful of oats then remove your fist while turning the stocking inside out. Tie a knot to form an oatmeal ball then cut and repeat until you have approximately 10-12 oatmeal balls. Store in a dry place and keep on hand to help relieve some of the symptoms associated with eczema, dermatitis and psoriasis such as itching, burning and tightness.
These oatmeal balls can be placed in a warm bath or for more concentrated oatmeal water, placed in a basin of warm water. The soothing 'milky' water can then be wiped over the skin with cotton wool or a muslin cloth.
NEVER allow moisture to evaporate on the skin. Apply Pure Jojoba straight away. This helps to lock in much needed moisture to hydrate and relax the surface of the skin. "
Re cats and essential oils - cats metabolise essential oils very differently to humans, I'll try and dig up the research. Essential oils used on cats very very quickly reach toxic levels and extreme care should be taken. Dogs metabolise essential oils similarly to humans, not nearly as much of a concern.
***added*** dug up some info on cats and essential oils http://www.aromaticsage.com/wary.htm